1. Field of the Invention
The present invention relates to a method and apparatus for the in situ rate and depth monitoring of thin film materials undergoing small thickness changes and, more particularly, to a method and appartus for the in situ monitoring and measurement of rate and depth of change in film thickness caused by etching or deposition.
2. Description of Problem and Prior Art
Typical fabrication techniques for microelectronic devices require small and exacting changes be made in thin films of material such as thin films of silicon material. Because conventional materials, such as silicon, used for such fabrication are often generally opaque to light because of overall thickness, conventional thin film interference techniques cannot be used to monitor the required small changes in thickness which result during device fabrication from etching or deposition.
Prior art techniques, such as those employed by Habeggar et al. in U.S. Pat. No. 4,147,435 typically rely on the use of two-beam interferometry to extract the etch rate of such opaque materials. In such arrangements, a single wavefront of light is split into two beams, one of which strikes a non-etching surface which serves as a reference while the other beam strikes the surface being etched which is moving away from the reference. The change in path length causes a phase shift which produces the sinusoidal-type signal after the two beams are recombined at the photo-detector.
The difficulty with two-beam interferometry approaches is that they all suffer from inherent poor thickness resolution since, in many instances, the total etch depth (such as in etching silicon) is on the same order of mangitude as the resolution, i.e. about .lambda./2 since the index of refraction is 1 in the case of, for example, Habegger et al. In addition, the typical two-beam interferometry approach suffers from the fact that the hardware is difficult to align and maintain aligned. Moreover, two-beam interferometry approaches suffer from poor signal-to-noise ratios.
A further difficulty with two-beam interferometry approaches resides in the fact that these techniques use photoresist (transparent) as a mask to create a non-etching reference surface. The techniques, then, are useful only when the resist available etches slower than the material being etched by a factor of about 3 or 4, for otherwise the resulting interferometry signal is ambiguous. One solution to this latter difficulty is to use non-erodable material as a mask, such as the Al.sub.2 O.sub.3 mask used by Logan et al as described in an article entitled "In Situ Rate Monitor/Etch End-Point Detector for Opaque Materials" in the IBM Technical Disclosure Bulletin, Vol. 21, No. 6, November 1978, p. 2314. While this latter approach remedies the erosion of the non-etching surface problem, it requires the introduction of foreign materials which may act to contaminate the product being manufactured.